The present disclosure relates to the aforementioned wrapped objects and in particular the use of removable and re-useable mandrels for manufacturing such objects. The manufacture of such wrapped objects comprises arranging, e.g. winding, one or more elongate elements, such as fibers, multi-fiber filaments, yarns, tapes, and/or so-called pre-pregs over a mandrel and bonding the elongate elements together to form the wrapped object. The objects may be large-size pressure vessels and/or complex-shaped objects from composites.
Bulk containers and tanks made from composite, fiber reinforced materials present cost-effective alternatives for metal vessels and tanks, e.g. because of their comparatively lower weight transport costs may be reduced. Manufacture of robust but thin-walled hollow metal objects such as metal tanks, in particular steel tanks, also tends to be more complex and expensive than that of wound tanks.
For manufacturing efficiency and costs, as well as for reasons of mechanical properties such as strength and robustness, wrapped objects and in particular wound tanks should be made in a single winding operation, i.e., the object should not be composed of separate segments attached together but be made as one integral object.
FIG. 1 indicates an exemplary stage in such manufacturing process for an elongated bulk tank vessel which comprises a cylindrical segment and dome-shaped end segments opposite each other. FIG. 1 shows a mandrel 1, a filament feeder, a rotary drive 3 for rotating the mandrel 1 with respect to the filament feeder, and first and second fiber bands 4 and 5, respectively. For manufacturing, the mandrel 1 is wound in filaments or tapes of pre-impregnated fibers. The fibers are applied in first bands 4 oriented generally in longitudinal direction of the tank, so-called “helicals”, and in second bands 5 oriented generally in azimuthal direction relative to the longitudinal direction of the tank, so-called “hoops”. Once the filaments or tapes are wound and bonded together a continuous and uninterrupted structure is formed around the mandrel, forming the tank wall, wherein the fibers or fiber bands are oriented in a desired way for withstanding forces of use of the tank.
In FIG. 1 the transition segments between the cylindrical segment and each end segment are filleted in accordance with force distribution in the end product and in accordance with an acceptable bending radius of the fibers during manufacture and use in the end product. Such transition segments may be considered joints between the adjacent segments and they are of particular concern hereafter.
The manufacturing processes requires a large number of process steps, thus being complex and expensive. Particular problems arise at the transition segments between the cylindrical segment and the end segments as set out below.
Manufacturing of a hollow object by wrapping e.g. a wound vessel over a mandrel produces a pressure force normal to the surface of the mandrel and directed inwards due to the tension on the fibers or tape at application. The pressure force increases with each layer of fibers added over the mandrel. The mandrel must withstand such pressure. In a foam mandrel the compression modulus and compression strength are generally decisive. In an inflatable mandrel the inflation pressure and possible deformation pressure are generally decisive.
The magnitude and direction of the force on any position on the mandrel surface are determined by the curvature of the surface: the stronger the curvature (i.e. the smaller the radius of curvature), the larger the local force. Thus, large forces may arise at a transition between two (curved) segments arranged at an angle to each other thus producing a bend section between the two curved segments over which fibers are wound at an angle to the direction of extension of the bend (i.e. a bending line) close to perpendicular. In vessels of the aforementioned type and shown in FIG. 1, this is the case for the “helicals” at the transition sections between the cylindrical segment and the domed end caps.
FIGS. 2A-3 indicate the effect of the radius of curvature at and near transition segments in different shapes of a wound tank over corresponding mandrels, indicated with 1 and drawn in full line and, respectively, indicated with 1′ and drawn in dashed line respectively. In the following, similar elements and details of the respective mandrels will be indicated with similar symbols, where expedient identified with or without a prime.
The mandrels 1, 1′ each are elongated along an axis A, A′. FIG. 2A is an axial longitudinal cross section view and FIG. 2B is a transverse cross section view as indicated in the respective Figs. with IIA and IIB, respectively. Note that in FIG. 2A the two different shapes under consideration are overlaid to show the differences; such differences are not visible in FIG. 2B. FIG. 3 is a detail of (the mandrels of) FIGS. 2A-2B
The mandrels 1, 1′ each comprise a cylindrical section 6, 6′ and (outwardly) convex end sections 7, 7′ so that the end segments of the formed tank would have a dome shape. The mandrel 1 further comprises a transition section 8 between the cylindrical section 6 and the respective end sections 7.
The cylindrical sections 6, 6′ of each mandrel have a substantially circular shape with radius R=R′ about the axis A (see FIG. 2B).
The end sections 7′ of the mandrel 1′ are hemispherical with a sphere radius R′ corresponding to the circle radius R′ of the cylindrical section 6′.
The end sections 7 of the mandrel 1 are flattened compared to a hemispherical shape, having a central portion which has a bending radius R″ significantly larger than the circle radius R of the cylindrical section 6, here being about 2.5 times as large, but it may have any radius of curvature from about two times as large up to orders of magnitude larger for tanks with very flat domes. The transition sections 8 on the other hand form a fillet between the cylindrical section 6 and the respective end sections 7 with a radius of curvature significantly less than the circle radius R, which may vary over the transition sections 8. The transition sections 8 have a common tangent with the first segment and a common tangent with the second segment, respectively, which tangents extend in different directions within one plane, thus, the transition section 8 forms a smooth variation between the cylindrical section 6 and the respective end sections 7 with a continuously varying tangent over the curvature (e.g. no discontinuities in spatial first derivatives of the normal of a tangent plane to the surface from the cylindrical section 6 to the respective end sections 7 across the respective transition sections 8, the second spatial derivative of the tangent within the plane preferably being smooth and nonzero).
In an inflated inflatable mandrel, the end sections will want to balloon and take on a (hemi-) spherical shape, such as in mandrel 1′. In an inflatable mandrel designed for another shape, e.g. mandrel 1 with relatively flattened end sections 7, this will lead to a non-uniform pressure build-up in the mandrel when inflated, with relatively high outward pressure in on the end sections close to the axis A and less pressure or even under pressure in the transition sections 8. This is indicated with arrow heads in FIG. 3. This may even lead to deformation of (the transition sections 8 of) the mandrel 1 from the intended shape to another shape, e.g. with caved in transition sections 8. Thus an object wrapped over such mandrel will have sub-optimally defined and supported transition segments. This effect is aggravated under the force of the filaments (tapes, fibers, . . . ), in particular “helicals” wrapped around such mandrel for manufacturing a wrapped object.
The present disclosure addresses this issue.
Further, in bulk storage tanks, in particular for liquids and/or foodstuffs, improved control over storage parameters of the interior volume of the hollow object may be desired. That aspect is addressed as well in the present disclosure.
Also, in particular for large-volume transport containers, structural integrity and robustness of the object is very important and improvements are sought after. That aspect is addressed as well in the present disclosure.